The invention relates generally to systems and methods for de-interlacing interlaced content for display on a progressive screen and more particularly to systems and methods for determining whether fields of an interlaced input stream can be paired, or grouped, without undesirable artifacts, such as systems and methods for determining whether an interlaced input stream was originally non-interlaced information and has undergone non-interlaced to interlaced conversion or systems that perform pattern recognition to facilitate suitable de-interlacing.
Interlaced content is typically video and is defined by standards such as NTSC, PAL or SECAM. Interlaced video may also be compressed video, such as that defined in the MPEG-2 standard for compressed video. If pairs/groups of even and odd fields can be combined without artifacts, then it is desirable for the image on the progressive display to be derived from a pair or group of fields rather than from just a single field. This generally results in a higher vertical resolution.
Pairing or grouping of two fields of video generally means making an association between two or more fields of video. Such an association may be used to combine fields and or weave even fields and odd fields within a pair or group together, either in whole or in part. When presented on a progressive (non-interlaced) display, a weaved image will have even field lines alternating spatially with odd field lines. In contrast, on an interlaced display, even field lines generally alternate spatially and temporally with odd field lines.
Unintelligent pairing and weaving of fields of video data can produce undesirable motion artifacts. These artifacts typically occur if motion is occurring in the video sequence, and if the paired fields of video were captured at significantly different points in time. The artifact can appear as a jagged edge or double image. Therefore it is desirable to improve image quality and intelligently pair fields and avoid motion artifacts. This includes content containing scenes with no motion, and content that was captured or created in a sufficiently non-interlaced way.
For example, an animator who creates an animated sequence may draw only one image for every five fields of video due to the effort involved. A progressive-to-interlaced conversion occurs when the animated sequence is transferred to a video format. To provide high quality images, it would be desirable to have a system that could automatically obtain knowledge of which fields of video were likely derived from the same original image so that proper pairing and display of the images results in removal of motion artifacts.
Also, as shown in FIG. 1, filmmakers have standardized on 24 film images per second. Film, when converted to NTSC undergoes a 3:2 pull down conversion. When film is converted to PAL, it undergoes a 2:2 conversion. Hence, with progressive display screens, such as computer screens and other screens that do not display images by altering between even and odd fields, interlaced images have to be de-interlaced for improved display quality. If the display system can detect a pattern in the image stream, such as a progressive to interlaced conversion, the system can better prepare images for the progressive display by combining fields that can be combined to create a higher quality image.
Moreover, other types of video content can be improperly displayed with motion artifacts resulting in less than desirable image quality. For example when video stream content is slow motion replay, such as from a sporting event, the content may have sets of successive fields that came from the same original captured image. Improper pairing of slow motion content can result in motion artifacts. Also, where little motion exists, the absence or partial absence of motion in a scene can result in undesirable artifacts, therefore parts of the scene should be properly de-interlaced to reduce such effects. In addition, if an image is paused, or advancing slowly forwards or backwards, such as from video playback devices or other source, typical default techniques of de-interlacing (referred to as "hop") can produce artifacts. A "hop" de-interlace technique, or two field bob, derives a progressive image form a single field by scaling and shifting the field. Usually the most recent field is used and this field will alternately be an even field, then an odd field, and then an even field. Another technique used, called "skip" or single field bob, will only use even or odd fields and will not alternate between the two fields. Therefore, suitable detection and pairing is needed to properly display such content.
In encoded video systems such as MPEG-2 systems, an interlaced video stream can contain precoded pull down status data that is analyzed by a decoder to determine whether the interlaced video stream has undergone pull down conversion. Conversion status data typically represents how the non-interlaced to interlaced conversion was done or is data the represents how the conversion should be undone. The decoder can then direct the field data from a frame storage device in a way that allows even and odd fields from the same film frame to be combined and displayed in a proper order.
However, most interlaced input video streams do not contain precoded pull down conversion status data or any other kind of pre-coded data information that indicates that a particular pattern exists. It is difficult to achieve a higher quality de-interlaced image since the decoder and de-interlacing system does not know which fields can be paired or grouped without artifacts and which fields cannot. As a result of the absence of the pre-coded information, video streams are typically not de-interlaced by properly pairing fields.
Consequently, there exists a need for a system and method for determining how to effectively de-interlace based on pattern analysis or other image evaluation technique. Where the content in the video stream underwent conversion, the system should reconstruct non-interlaced captured content for display on a progressive screen by automatically determining whether an interlaced input video stream has fields that can be paired or group to improve image quality. Such a system and method should determine whether the video content was originally recorded in a non-interlaced fashion and has undergone non-interlace to interlace conversion or the whether another type of image pattern exists in the content to suitably de-interlace the video content. Such as system and method should make the determination in an expedient manner to facilitate suitable image resolution on a progressive display.